Electron-beam induction accelerators, such as linear induction accelerators (LIA), are typically constructed in a modular multi-stage manner with each stage developing an increment of the total accelerator voltage. This enables the use of many small accelerating voltages instead of one very large one to confine the total acceleration voltage and suppress breakdown, losses, and electromagnetic interference. One particular type of pulse-power architecture for driving modules in LIAs is the Blumlein pulse generator comprised of parallel-plate transmission lines and arranged in stacked configuration. Whether driven by stacked Blumleins or other pulse-power architectures, however, the multi-stage induction accelerator structure is often enclosed in a grounded metal enclosure so that the full beam voltage (which for many applications is in the Megavolt range) is not developed on the exterior of the machine. This is typically accomplished using magnetic materials such as ferrite or tape-wound magnetic alloys to increase the inductance of the short-circuit created by the metal case surrounding each accelerator stage, and prevent the external metal case from shorting the accelerating field. Such magnetic core materials, however, are heavy, bulkyw, expensive, nonlinear, and electrically lossy for the fast, nanosecond-range pulses of interest in many applications, and are therefore generally undesirable.
Various core-free pulse architectures are known. One class uses pairs of lines with widely different dielectric constants while another class uses combinations of open-circuit lines combined with short-circuit lines. In either case these designs are encased in metal and support stackable output pulses without the need for magnetic isolation cores. These configurations are known as bipolar or zero-integral configurations because they produce a positive and negative voltage pulse with a net time integral of zero, i.e. having equal and copposite voltage-time products. The zero voltage-time integral that results means that zero net magnetic flux has been induced in the cavity. Acceleration of the beam takes place on the second part of the pulse and can use all of the available energy. Such bipolar pulse generation structures are typically presented in the literature as generic transmission line diagrams that could be realized in either coaxial or planar geometry.
One example of such a bipolar pulse forming line is shown in the publication, “Linear Induction Accelerators Made from Pulse-Line Cavities with External Pulse Injection,” by Ian Smith (Rev. Sci. Instr., vol. 50(6), pp. 714-718, 1979), incorporated by reference herein. FIGS. 2a-d of that publication show several exemplary bipolar transmission line configurations. In particular, the transmission line structure shown in FIG. 2d is reprinted and shown in FIG. 1 of the present drawings at reference character 100. It is shown having an upper conductor 101, a middle conductor 102, and a lower conductor 103. The middle conductor 102 is actively chargeable to a voltage V0, while the outer conductors 101 and 103 are ground conductors. Dielectric material/media 108, 109 fill the cavities between the conductors 101-103, with both characterized by impedance Z0. A first end 105 of the formed stack is the pulse output end, e.g. adjacent an acceleration axis for LIA applications, and a second end 106 of the stack is opposite the first end. At the second end 107, the first conductor 101 and the third conductor 103 are electrically connected via a passive, short-circuit line (“shorted-line”) 104 extending away from the second end 106 in an opposite direction of the first end 105, and having dielectric media 110 which together form a passive shorted section also characterized by impedance Z0. And a switch region 107 is shown at the second end 106 to discharge the energy stored in the second conductor 102 to the third conductor 103.
When switched, this structure 100 produces a bipolar pulse of ±V0 when charged to V0 into a matched load (not shown) of 2Z0 that appears at the second half cycle. Naturally, the structure is fully encased in metal. The shorted-line represented by conductor 104 and dielectric 110 on the left side is passive in that it is not charged. In this manner, the cavities of this structure are shaped internally as constant impedance transmission lines to generate rectangular flat-topped acceleration pulses with a constant current beam load. Moreover, the transmission line circuits produce voltage waveforms that are bidirectional and have zero time integral of voltage when driving a matching load.
The configuration of this type of structure can be problematic, however, because the pulse-forming switches are inside the structure cavity and not easily accessible for triggering. It would be advantageous to provide a bipolar, zero-net-time-integral pulse generator design for use in an LIA that is stackable, does not require a magnetic core, and would allow reasonable engineering access to the switches that are always required in real systems.